1. Field of the Invention
The present invention relates to a capacitance type seat occupant sensor system to be installed in a vehicle.
2. Description of the Related Art
JP-Hei-11-271463-A discloses a seat occupant sensor system that includes a capacitance type sensor and an electronic control system. The capacitance type sensor includes a main electrode disposed inside a seat of a vehicle to detect a change in an electric field that is formed between the main electrode and a portion of a vehicle body.
If no occupant sits on the seat, there is nothing but air in the electric field. If there is a member of a child restraint system (CRS) on the seat, the member intervenes in the electric field. If a passenger sits on the seat, a human body intervenes in the electric field. The dielectric constant of the air is about 1, the dielectric constant of the child seat is usually between 2 and 5, and the dielectric constant of the human body is about 50. Therefore, the capacitance of the capacitor formed between the seat and the vehicle body changes as the occupant of the seat changes among air, the child seat or the human body. Thus, the occupant sensing system discriminates the occupant based on the change in the capacitance.
An air bag system operates according to the result of the discrimination. If, for example, there is no occupant on the seat, an air bag is prohibited to operate. On the other hand, the air bag is allowed to inflate if there is an adult person on the seat.
Because the dielectric constant of the water is 80 and close to the dielectric constant of the human body, it is difficult to discriminate the human body without additional sensor if the seat is wetted by water. In order to discriminate the seat occupant from the wet seat, JP-2006-27591-A proposes a capacitance type sensor system that employs an additional sensor to detect a wet condition of a seat.
JP-2006-201129 discloses a capacitance type occupant sensor system that increases the amount of current flowing between a pair of electrodes only when there is an occupant on the seat.
JP-2006-27591 discloses a capacitance type occupant sensor system that includes a guard electrode for detecting an abnormal condition of a capacitor formed between the guard electrode and the main electrode.
Such capacitance type occupant sensor systems detect an occupant by means of voltage of the capacitance or the amount of current flowing between a pair of electrodes of the capacitance. The amount of current is related to a component intervening between the electrodes and resistance of the circuit that forms the capacitance type occupant sensor. In other words, such resistance includes the resistances of the human body, water, air, etc. These resistances are connected in parallel to the capacitance.
Therefore, the detected amount of current includes an amount of current flowing between the pair of electrodes and an amount of current flowing through the human body, water, air etc. Therefore, it is difficult to discriminate an occupant on the seat with high accuracy.